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How to Find the Radius from the Circumference (Mathematics Lesson)

What Is the Radius in Terms of the Area?

The radius of a circle is the square root of the area of a circle divided by π.

How to Find the Radius from the Area

Question: What is the radius of the circle, with area A, below?



Step 1: Divide the area, A, by π.
A ÷ π = Aπ

Step 2: Take the square root.
Aπ

This is the radius, r, of the circle.

A Real Example of How to Find the Radius from the Area

Question: What is the radius of the circle, with area 10cm2, below?



Step 1: Divide the area, 10cm2, by π.
10 ÷ π = 10 ÷ 3.14 = 3.18cm2

Step 2: Take the square root.
√ 3.18 = 1.78cm

The radius of the circle is 1.78cm.

Another Real Example of How to Find the Radius from the Area

The slider below shows another real example of how to find the radius of a circle from the circumference:
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Note
WHAT IS A CIRCLE?

A circle is a shape containing a set of points that are all the same distance from the center. This distance is the radius.